1. Field of the Invention
This invention relates to multicolor liquid toning of electrophotographic images, and particularly to processes of liquid toner development where two or more toner images of different colors are first superimposed and then simultaneously transferred to a receptor surface.
2. Background to the Art
Early toning of electrophotographic imaging was performed with toner powders even though benefits were recognized in the use of liquid toners.
Metcalfe & Wright (U.S. Pat. No. 2,907,674) recommended the use of liquid toners for superimposed color images as opposed to the earlier dry toners. These liquid toners comprised a carrier liquid which was of high resistivity e.g., 10.sup.+9 ohm-cm or more, colorant particles dispersed in the liquid, and preferably an additive intended to enhance the charge carried by the colorant particles. Matkan (U.S. Pat. No. 3,337,340) disclosed that a first deposited toner may be sufficiently conductive to interfere with a succeeding charging step. He described the use of insulative resins (resistivity greater than 10.sup.-10 ohm-cm of low dielectric constant (less than 3.5) covering each colorant particle. York (U.S. Pat. No. 3,135,695) disclosed toner particles stably dispersed in an insulating aliphatic liquid, the toner particles comprising a charged colorant core encapsulated by a binder of an aromatic soluble resin treated with a small quantity of an aryl-alkyl material. The use of explicit dispersant additives to the toner dispersion is disclosed in U.S. Pat. No. 3,669,886.
The use of polyvalent metal soaps and blends thereof for improved liquid toner conductivity characteristics is disclosed by Hochberg (U.S. Pat. No. 3,890,240). The characteristics included latititude in concentration addition of the charging agents and improved density uniformity. The application was for liquid toners used in electrostatic photocopying. U.S. Pat. No. 4,707,429 described polyvalent metal soaps which are dispersed with thermoplastic resin binders for improved imaging characteristics by affecting the charge direction.
In U.S. Pat. No. 4,891,286 liquid toner mobility was found to be increased by the addition of insoluble monomeric organic acids and this was found most advantageous for high speed copying purposes. The toner particles described were negatively charged particles. U.S. Pat. No. 4,026,789 teaches the use of a variety of carrier soluble organic acids that enhance the positive charge of toner particles.
Improvements in copier performance have been described in U.S. Pat. No. 3,681,243 when the liquid toner contains at least one of the groups consisting of metal dialkyldithio-phosphates, sodium alkyl-phosphates, alkyl phosphates, alkali metal-alkyl sulphates, alcohols, monocarboxylic acids, phthalic acid, alkyl phthalates, ammonia, amines, aldehydes, and styrene.
The advantages of using binders comprising organosols (sometimes described as amphipathic particles) are disclosed in patents assigned to Philip A. Hunt Chemical Corp. (U.S. Pat. No. 3,753,760, U.S. Pat. No. 3,900,412, U.S. Pat. No. 3,991,226). Amongst the advantages is a substantial improvement in the dispersion stability of the liquid toner. The organosol is sterically stabilized with a graft copolymer stabilizer, the anchoring groups for which are introduced by the esterification reaction of an epoxy (glycidyl) functional group with an ethylenically unsaturated carboxylic acid. The catalyst used for the esterification is lauryldimethylamine or any tertiary amine. A similar treatment is found in U.S. Pat. No. 4,618,557 assigned to Fuji Photo Film Co., except that they claim a longer linking chain between the main polymer and the unsaturated bond of the stabilizing moiety. The comparative examples with the Hunt toners show improved image quality found over the Hunt toners due to image spread. They ascribe the improvement to the use of the longer linking chains. In both the Hunt and the Fuji patents charge director compounds, when used, are only physically adsorbed to the toner particles.
Diameters of toner particles in liquid toners vary from a range of 2.5 to 25.0 microns in U.S. Pat. No. 3,900,412 to values in the sub-micron range in U.S. Pat. No. 4,032,463, U.S. Pat. No. 4,081,391, and U.S. Pat. No. 4,525,446, and are even smaller in a paper by Muller et al, "Research into the Electrokinetic Properties of Electrographic Liquid Developers", V. M. Mueller et al, IEEE Transactions on Industry Applications, vol IA-16, pages 771-776 (1980). It is stated in U.S. Pat. No. 4,032,463 that the prior art makes it clear that sizes in the range 0.1 to 0.3 microns are not preferred, because they give low image densities.
Liquid toners that provide developed images which rapidly self-fix to a smooth surface at room temperature after removal of the carrier liquid are disclosed in U.S. Pat. No. 4,480,022 and U.S. Pat. No. 4,507,377. These toner images are said to have higher adhesion to the substrate and to be less liable to crack. No disclosure is made of the use in multicolor image assemblies.
In the toners disclosed in the U.S. Pat. No. 3,753,760, U.S. Pat. No. 3,900,412, U.S. Pat. No. 3,991,226 (the Hunt patents), the presence of a few parts per million of a tertiary amine in the liquid toner medium produces toners with very high conductivity especially when the toner is charged with a metal soap. This causes flow of the toner during imaging which in turn degrades the image. The high conductivity is derived from the protonation of the tertiary amine groups by the unsaturated carboxylic acid groups, thus giving ionic carriers in the liquid. Another problem associated with the use of tertiary amine is the high background in the non-imaged areas which is the result of negatively charged or non-charged particles. The esterification reaction of the glycidyl groups and the carboxylic groups usually does not go to completion under the reaction conditions for making the organosol. The examples in these patents show that between 25% to 50% of the carboxylic acid groups could be esterified. In other words about 50% to 75% of the carboxylic acid still remain in the dispersion medium. During the dispersion polymerization reaction for making the latex, the unreacted unsaturated acid can copolymerize with either the core part of the particle or the stabilizer polymer or both at the same time. The tertiary amine also may become attached onto the polymer particle by hydrogen abstraction. The presence of carboxylic acid on the particle and tertiary amine in the liquid medium or on the particle would be expected to result in the formation of carboxylic anions on the particle which is a good source for a negative charge.
These problems have been eliminated from our toner through the use of a suitable catalyst other than tertiary amines or the use of other anchoring adducts that can be catalyzed with catalysts other than tertiary amines.
U.S. Pat. No. 4,618,557 draws attention to the poor performance of the prior art (Hunt) toners and relates it to the number of carbon atoms in the linking chain. We have found that the use of a tertiary amine catalyst for attaching an unsaturated group to the main chain of the stabilizing resin via linking groups is the main reason for the poor performance of Hunt's liquid developers. It is believed therefore that the liquid developers of U.S. Pat. No. 4,618,557 showed better quality images compared with Hunt's because they do not use a tertiary amine catalyst, rather than the claimed use of long linking groups. However, that patent failed to disclose anything related to the present invention. Toners according to the present invention are superior to the toners of U.S. Pat. No. 4,618,557 for these reasons:
a) The prior art patent uses zirconium naphthenate as the charge director for their liquid toners. The metal cation is physically adsorbed onto the dispersed particles. This method usually results in a charge decay with time due to the gradual desorption of the metal soap from the particles. Toners according to the present invention do not suffer a charge decay because they are charged with metal chelate groups chemically attached to the resin particles.
b) U.S. Pat. No. 4,618,557 uses mercury acetate, tetrabutoxy titanium or sulfuric acid as catalyts for the anchoring reaction. Some of the substances are toxic (such as mercury acetate) and must be removed from the toner. However, the patent uses subsequent steps to remove the catalysts by precipitation from a non-solvent such as acetonitrile or methanol. These solvents may be trapped in the stabilizing polymer and are very difficult to remove. The present invention selectively chooses catalysts and reactants so that there is no need for the purification step.
The toners disclosed in U.S. Pat. No. 4,564,574 are based on chelating polymers containing cationic groups neutralized with counter anions as the source of the charge. The polymer may be a homopolymer, copolymer, block copolymers or graft copolymer comprising a coordinating compound bound to the backbone of the polymer. The chelating polymer is prepared in solution by a free radical polymerization reaction (using DMF as the solvent). After precipitating the polymer and redissolving it in a suitable solvent (THF), it is allowed to react with a metal cation. Those toners are prepared by milling a solution of the polymer in a suitable solvent (THF) with a pigment. The ratio of pigment to polymer is 1:4. Through this process, the polymer is adsorbed onto the surface of the pigment particles. Finally the blend is diluted with Isopar.TM. G to the proper concentration.
The polymers of U.S. Pat. No. 4,564,574 are prepared in a liquid medium which is a good solvent for the polymer, whereas our chelate polymers, are prepared by dispersion polymerization techniques wherein the liquid medium is not a good solvent for the dispersed polymeric particles. It is also well known that conducting a metal chelate reaction of a transition metal cation and a polymer containing coordinating groups in a liquid, which is a good solvent for the polymer, results in the formation of a crosslinked metal chelate gel. Some coordinating compound groups can lose a proton when they form ligands with a transition metal cation. This proton can neutralize the anion of the metal cation, thus reducing the overall charge of the material, which would be expected in the practice of the technology of that patent. The resulting metal chelate complex does not dissociate in a hydrocarbon solvent system.
Also, that patent claims that the use of a coordination compound in combination with any neutralizing anion such as halide, sulfate, p-toluenesulfonate, ClO4.sup.-, PF6.sup.-, TaF6.sup.- or any relatively large anion, would improve the dissociation of the corresponding ion pair in an apolar medium. Transition metal complexes or salts of these anions usually do not dissolve in a hydrocarbon liquid such as Isopar.TM. G. It is not apparent how they could dissociate in such a non-solvent system to give the charge on the particles necessary for good electrostatic imaging. The physical results in practice, showing low Zeta potentials for toner according to that invention, substantiate this analysis.
The toners of the present invention are based on polymer dispersions which are prepared by dispersion polymerization techniques in an aliphatic hydrocarbon liquid. The polymer dispersion consists of pendant chelate groups attached to the soluble polymeric component of the particle. This component consists of a graft copolymer stabilizer containing metal chelate groups. The stabilizer polymer is chemically anchored to the insoluble part of the polymer (the core). Since these particles are in constant movement, cross-linking through the metal complex would be very difficult. In some cases cross-linking may take place in latices with high solid contents (&gt;10%) due to the close distance between the particles. However, in latices with solid contents of less than 10%, cross-linking does not occur and the 1:1 complex is formed. In such a case only one counter ion (anion) of the metal salt is neutralized, while the other anions are still bound to the transition metal atom and dissociate in a hydrocarbon liquid. The new metal chelate latices of the present invention have been found to dissociate in a hydrocarbon liquid to give a high charge on the dispersed particle.
In U.S. Pat. No. 4,798,778 a liquid electrostatic developer containing modified resin particles are described. Also described are several procedures for preparation of the liquid developers which contain the resin particles.
The resin particles consist primarily of ethylene homopolymers or copolymers with certain types of esters, where the esters have certain substituents, e.g., hydroxyl, carboxyl amine, and acid halide. The resin particles once formed have an average particle size of less than 10 um.
The process for preparation of developers with the resins include mixing with the nonpolar fluid (Isopar.TM. G) at an elevated temperature to liquify the resin, cooling the formed particles, reacting the suspension with compounds selected from alkyl amine, alkyl hydroxide amino alcohol, etc., and adding charge control agents to the suspension. The resultant toners carry a net negative charge as described in U.S. Pat. No. 4,798,778.
There are several differences between the present invention and the described patent including the solubility of the added resinous material, and the polarity of the resultant liquid electrostatic developer, and the less complicated procedure of simply incorporating the described material during milling.